1. Field of the Invention
This invention involves improvements to communications systems and methods in a wireless communications system.
2. Description of Related Art
Adaptive beamforming technology has become a promising technology for wireless service providers to offer large coverage, high capacity, and high quality service. Based on this technology, a wireless communication system can improve its coverage capability, system capacity, and performance significantly.
The personal wireless access network (PWAN) system described in the referenced Alamouti, Stolarz, et al. patent applications, uses adaptive beamforming combined with a form of the CDMA protocol known as discrete multitone spread spectrum (DMT-SS) to provide efficient communications between a base station and a plurality of remote units (RU).
An orthogonal frequency division multiplex (OFDM) waveform is composed of many closely spaced carriers, each carrying a single complex (magnitude and phase) symbols. The OFDM carriers are chosen such that the lowest frequency carrier""s period is entirely the symbol time duration and each successive carrier is an integer multiple of that frequency. Prior to transmission, the composite signal consisting of multiple orthogonally spaced tones each carrying a single information symbol, is converted into the time domain via an inverse fast Fourier transform (FFT) and transmitted as a complex time domain waveform with a symbol duration as defined above.       (          1              Δ        ⁢                  xe2x80x83                ⁢        fcarrier              )    .
Since each carrier (referred to as tone) is modulated by an individual symbol from a users data, the phases are random. This condition can be assured with data scrambling or pre whitening techniques to assure random phase (and possibly amplitude) distribution during idle data streams. The time domain transmitted waveform is thus very noiselike with a peak to average ratio determined by the number of tones and their randomness.
Forward link data (from central hub or base) must contain synchronization information such that remote stations can synchronize in time and frequency to their serving base station. The receive window at each remote station must be adjusted as closely as possible to the received symbol packet (including time of flight delays) to minimize phase change across frequency in the received symbol set. In addition it is desirable to derive system clock and timing information from the base station.
Reverse link transmissions from the remote station to the base station must be received from multiple users within a fixed receive synchronization preamble for the OFDM waveform""s window at the serving base station. Errors in transmit timing will result in signals arriving early or late at the desired base station. Either case will yield a phase ramp (either positive or negative) on the received data symbols. Large timing errors will result in partial sampling of incoming time domain waveforms and a resulting loss of orthogonality. In that case received packets that have timing errors will cause large scale interference to all correctly synchronized users.
What is needed is a method to assure accurate synchronization of both forward and reverse links in an OFDM system.
A highly bandwidth-efficient communications method is disclosed that enables remote stations to synchronize in time and frequency to their serving base station. The invention enables a base station and its remote stations in a cell to synchronize in a noisy environment where signals interfere from other base stations and remote stations in other cells. The base station forms a forward synchronization burst that includes a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern unique to the base station. The unique pattern enables a remote station to distinguish the base station""s bursts from other signals present in a crowded area. The distinctive orthogonal frequency division multiplexed pattern can be a Hadamard code pattern, for example. When the a base station has received a signal on a reverse link from a remote station, having significant interference, the base station selectively forms a request signal requesting the remote station to respond with a reverse synchronization burst that includes a plurality of tone frequencies arranged in the same distinctive orthogonal frequency division multiplexed pattern. The base station then transmits the forward synchronization burst and the request signal at a base station reference instant of time to the remote station. The base station forms the synchronization burst by computing spreading weights to spread an outgoing synchronization signal over the plurality of outgoing synchronization tone frequencies, using the distinctive Hadamard orthogonal frequency division multiplexed pattern.
The receive window at the remote station is controlled by the remote station""s reference clock to open at a remote station reference instant before the expected time of arrival of the forward synchronization burst. The phases of signals received by the remote station are referenced with respect to the remote station reference instant. Later, when the remote station sends signals back on the reverse link to the base station, the instant of transmission is referenced with respect to the remote station reference instant. And the phases of signals transmitted by the remote station are referenced with respect to the remote station reference instant. Thus, any errors in the remote station reference instant impairs the SINR of both the forward and reverse links.
The remote station receives the forward synchronization burst and despreads the spread signal by using despreading weights. When the remote station receives the forward synchronization burst from the base station, it recognizes that its serving base station is the source of the unique pattern of the forward burst. Then, in response to the request signal accompanying the forward burst, the remote station prepares a reverse synchronization burst that includes a plurality of tone frequencies arranged in the same distinctive orthogonal frequency division multiplexed pattern. The unique pattern enables the base station to distinguish the remote station""s bursts from other signals present. The remote station then transmits to the base station on the reverse link, the reverse synchronization burst. The reverse synchronization burst includes an error signal transmitted at an instant referenced with respect to a remote station reference instant of time. To maximize the signal-to-interference-noise ratio (SINR), the base station monitors the time of arrival and phase of the signals sent on the reverse link from the remote station, to derive clock correction values that it then sends to the remote station.
The reverse synchronization burst that is received by the base station is in the form of a spread signal comprising an incoming signal that includes the synchronization signal spread over a plurality of incoming frequencies. The base station adaptively despreads the spread signal by using despreading weights, recovering the distinctive Hadamard orthogonal frequency division multiplexed pattern. The base station recognizes the reverse synchronization burst and derives a correction value from the error signal, related to a relative time error between the base station reference instant of time and the remote station reference instant of time. The relative time error is the difference between the base station reference instant of time and the remote station reference instant of time less a propagation duration of time of the synchronization burst from the base station to the remote station. The relative time error is compared with the desired relative time difference value. This is the difference between the base station reference instant of time and a desired remote station reference instant of time less the propagation duration of time of the synchronization burst from the base station to the remote station.
Then the base station transmits the correction value to the remote station to correct timing at the remote station. The base station computes spreading weights to spread correction value signals over a plurality of outgoing frequencies to be transmitted to the remote station. In a preferred embodiment, the base station is part of a wireless discrete multitone spread spectrum communications system. In another aspect of the invention, the reverse synchronization signals selectively occupy time slots in the transmission frame from the remote station to the base station, that would otherwise be occupied by channel control or traffic signals. Only when the base station requests the remote station to respond with a reverse synchronization burst, does this burst preempt the time slot from its other uses.
Currently, the invention has advantageous applications in the field of wireless communications, such as cellular communications or personal communications, where bandwidth is scarce compared to the number of the users and their needs. Such applications may be effected in mobile, fixed, or minimally mobile systems. However, the invention may be advantageously applied to other, non-wireless, communications systems as well.